John Wagner was ordered to serve an additional seven years in prison for the 2010 murder of a Johns Hopkins researcher after a judge ruled Friday that the stabbing death violated Wagner's probation from an earlier assault case, the Baltimore State's Attorney's Office announced.

Wagner, 38, was sentenced to life in prison plus 20 years in October for the robbery and murder of Stephen Pitcairn last year, as the young man walked home along St. Paul Street. Both he and his co-defendant, Lavelva Merritt, were on probation for other cases at the time of the attack.

Prosecutors asked that Wagner's probation violation sentence, handed down by city Circuit Court Judge John Addison Howard, be served consecutively to the others, ensuring that Wagner, 38, will spend decades behind bars.

"This proceeding reflects our commitment to using absolutely every tool at our disposal to ensure that repeat, violent offender remain in jail and off of our streets," Baltimore State's Attorney Gregg Bernstein said in a statement.

Merritt was sentenced to 15 years in prison last month for her role in the robbery. She is scheduled for a violation of probation hearing on Monday.